The invention relates to a rotor blade consisting of fiber-reinforced plastic for a wind power plant. To decrease radar reflection, it comprises a passive radar absorber that is integrated into the surface.
Flight safety radar systems, which as 2-D radar systems, require the direction and the distance of a flying target in order to display it. Due to the motion of the wind energy rotors, or the rotor blades, such flight safety radar systems generate a moving target. The rotor blades as moving targets are identified just like aircraft, by a sufficiently strong reflection with Doppler components. Thereby, additional moving targets are generated for the radar system, which impede or falsify the flight path of the actual flying target.
From earlier patent applications, for example, DE 199 29 081 A1, inference absorbers and Jaumann absorber designs are known for reducing radar reflection, which ensure a reduction of radar reflection that is of particularly high quality. Because of the very high radar cross-section of a wind power plant of more than 400 m2, however, adequate reflection dampening greater than 20 dB in the operating frequency range of radar systems for flight safety is not guaranteed in all atmospheric conditions. It is common to all previous solutions and production methods that as many surface sections as possible are provided with a reflection-dampening effect.
Exemplary embodiments of the present invention involve a specific use of a radar absorber to ensure that the radar system recognizes the wind power plant as such and is able to differentiate it from an actual flying target.
In particular, exemplary embodiments of the present invention involve a rotor blade for a wind power plant comprising fiber-reinforced plastic; and a radar absorber embedded in the fiber-reinforced plastic. The radar absorber includes a layer close to a surface of the rotor blade at a depth of 2 to 5 mm below the surface, the layer close to the surface having a defined electric sheet resistivity of 100 to 800 ohm/square, and a layer far removed from the surface of the rotor blade at a depth of 5 to 16 mm, the layer far removed from the surface having a defined electric sheet resistivity of at a maximum 50 ohm/square. The embedded radar absorber covers one or several discrete surface sections of the rotor blade, without covering the entire surface of the rotor blade.
In accordance with the invention, the passive radar absorber that is embedded in the fiber-reinforced plastic material of the rotor blade comprises the following elements:
a layer close to the surface, in particular a fibrous web, cloth, knitted fabric or a film. It has a defined electric sheet resistivity of 100 to 800 ohm/square and is located at a depth of 2 to 5 mm underneath the surface of the rotor blade;
a layer far removed from the surface layer at a distance to the layer close to the surface of 5 to 16 mm. The layer far removed from the surface layer can also be designed as fibrous web, cloth, knitted fabric or film. The layer far removed from the surface is a technically conductive layer with a defined electric sheet resistivity of at a maximum 50 ohm/square.
The integrated radar absorber is limited selectively to one or more discrete, i.e. surface areas of the rotor blade that are separated from each other, without covering the entire surface of the rotor blade.
Thereby, the surface areas at the rotor blade edges are preferred, whereby the absorber can also be limited exclusively to surface sections at the rotor blade edges.
In one aspect of the rotor blade, a section of the rotor blade edges is located in at least one of the discrete surface sections at which the radar absorber is present.
In a further aspect, a section of rotor blade edges is present in all discrete surface sections at which a radar absorber is present.
This absorber design in accordance with the invention ensures a mono-static reduction of reflection within a limited angle of incidence.
As the radar absorber does not have to cover all surface sections that are relevant for reflection, this results in advantages with respect to the use of material and weight.
Due to the selective arrangement of the radar absorber on one or several isolated surface sections of the rotor blades, a defined chronologically dependent intensity of reflection is generated during a rotation of the rotor, the characteristic of which makes it possible for the signal processing system of a radar facility to identify this target as wind energy plant and to filter it out. The path formation or tracking of the actual flying target is thereby not impaired.
The quality of the dampening of the reflection by the radar absorber and the use of surfaces by the radar absorber with respect to the design, the dimensions of the rotor blade and the speed of rotation of the rotor make it possible to ensure a defined chronological dependence of the intensity of reflection in the direction of the radar facility.
For the fiber-reinforced plastic material of the rotor blade, the materials that are generally used as starting materials can be used (resin matrix or polymer matrix; carbon fibers, glass fibers or aramide fibers.).
In order to adjust the required electrical sheet resistivity of 100 to 800 ohm/square for the layer of the radar absorber near the surface, commercial fiber materials with low electric conductivity can be adapted correspondingly, for example, by weaving in metallic threads or by coating the fiber material with conductive materials.
In order to be effective for certain polarization orientations of the radar system, the layer of the radar absorber that is close to the surface can have an orientation on the surface, which brings about an orientation-dependent surface conductivity. This can, for example, be achieved by differences in the fiber density or in fiber diameter.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.